This invention relates to a weld gun for resistance welding fasteners to a metallic object, and more particularly, this invention relates to a fastener detection sensor.
Weld gun assemblies are used to resistance weld fasteners to metallic objects such as body panels for vehicles. A fastener such as a weld stud or nut are loaded onto one of the electrodes. The electrodes are moved together with an actuator placing the fastener into engagement with the panel under pressure. Very high currents are applied to the electrodes, which welds the fastener to the panel.
The use of weld guns is typically an automated process in which the fasteners are loaded into the weld gun assembly. Occasionally the fastener is not properly loaded into the weld gun or the fastener may not be loaded into the weld gun at all. Under these circumstances, it is important to detect that there is a problem with the fastener so that the fastener can be properly welded to the panel. To this end, sensors have been used in an effort to determine the presence and orientation of the fastener in the weld gun. Typically, a linear variable displacement transducer has been connected to an electrode arm to measure the movement of the arm. If no nut is present the arm will move a greater amount than if a fastener is present and in the proper orientation. If the fastener is not in the proper orientation, then the electrode arm will move slightly less than if the fastener is in the proper orientation. In this manner, fastener presence and orientation has been detected. However, it has been difficult to accurately detect the proper orientation of the fastener for several reasons. First, the position of the electrode arm does not always correspond to whether the fastener is in the proper orientation. That is, the location of the position sensor cannot detect fastener orientation accurately because of tolerance stack-ups and variations in the fastener. Second, the high electrode currents introduce inaccuracies into the position sensor signal. Even with magnetically shielded cables, enough interference from the electrode current is introduced to give false fastener readings. Therefore, what is needed is a sensor that more accurately detects the orientation and presence of a fastener in a weld gun assembly.